1. Field of the Invention
The present invention relates to new coating compositions containing chemically modified, amorphous polyolefins, particularly polypropylenes; a process for coating plastic substrates with these coating compositions; and to the coated plastics thus obtained.
2. Description of the Prior Art
To apply firmly adhering paint coatings to plastics, particularly to polypropylene (PP) injection-molded parts, the substrate surfaces have to be chemically or physically modified. Reference is made in this regard to flame treatment (cf. for example F. Garbassi et al, J. Mater. Sci., 22, pages 207 to 212 (1987)), corona treatment (cf. for example F. Lawson, Rubber Chem. and Technol., 60, 102 (1987)), plasma treatment (cf. for example L. H. Coopes et al, J. Macromol. Chem., 17, pages 217 to 226 (1982) and EP-A-0 282 094) and pickling with chromosulfuric acid (cf. for example M. Salkauskas, J. Appl. Polym. Sci., 26, pages 2097 to 2098 (1981)). It is also known from the literature that plastic parts based on polypropylene and polyethylene can be painted with firmly adhering coatings by the use of primers based on clorinated polyolefins U.S. Pat. No. 3,676,391) or polyolefins which are modified with maleic acid anhydride (DE-OS 1,546,982).
These methods are expensive and, in addition, cannot be applied to injection-molded parts of complicated geometry. Apart from the high handling costs attributable to their low solids content, application of the primer coatings mentioned above additionally involves environmental problems. In addition, their use is limited to certain paint systems.
According to published PCT application WO 88/05346, the primer coatings are modified by additional measures, such as UV irradiation, which involves additional expense.
The addition of standard chlorinated polypropylenes, to plastics paints, as recommended in U.S. Pat. No. 2,468,480, is attended by the disadvantage that chlorinated polyolefins of this type are not compatible with conventional plastics paints. In addition, their solubility in typical paint solvents is very poor, which adversely affects their processibility.
Accordingly, the problem addressed by the present invention is to provide a new process for painting plastics which, through the use of new adhesions promoters, is suitable for the production of high-quality coating compositions without being attended by any of the disadvantages of the prior art processes mentioned above.
It was possible to solve this problem in accordance with the present invention by the use of certain adhesion promoters as described in detail hereinafter.